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Dynamic Molecular Oxygen Production in Cometary Comae


Dynamic Molecular Oxygen Production in Cometary Comae

Yunxi Yao et al. Nat Commun.


Abundant molecular oxygen was discovered in the coma of comet 67P/Churyumov-Gerasimenko. Its origin was ascribed to primordial gaseous O2 incorporated into the nucleus during the comet's formation. This thesis was put forward after discounting several O2 production mechanisms in comets, including photolysis and radiolysis of water, solar wind-surface interactions and gas-phase collisions. Here we report an original Eley-Rideal reaction mechanism, which permits direct O2 formation in single collisions of energetic water ions with oxidized cometary surface analogues. The reaction proceeds by H2O+ abstracting a surface O-atom, then forming an excited precursor state, which dissociates to produce O2-. Subsequent photo-detachment leads to molecular O2, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. This abiotic O2 production mechanism is consistent with reported trends in the 67P coma and raises awareness of the role of energetic negative ions in comets.

Conflict of interest statement

The authors declare no competing financial interests.


Figure 1
Figure 1. Production of O2 and HO2 from energetic H2O+ bombardment of oxides.
Energy distributions of (a,c) O2 and (b,d) HO2 scattered from native Si oxide (a,b) and Fe oxide (c,d) following bombardment by H2O+ at various incidence energies. Physical sputtering contributions to the O2 signal become visible at high energies (E0>250 eV for SiOx, E0>150 eV for FeOy), when the dynamic O2 peak dies out.
Figure 2
Figure 2. Isotopic dosing experiments of H2O+ scattering on Pt covered with 18O atoms.
Energy distributions of ion exits of (a) 18O16O, (b) 16O16O, (c) 18O18O and (d) 18O from H2O+ scattering on Pt at various 18O2 exposure pressures, as annotated. The detection of 18O16O in a indicates fast O2 formation between 16O-atoms from H216O+ and 18O-atoms adsorbed on the Pt surface. The absence of 18O18O in c proves that fast O2 may not originate in the gas phase or from surface sputtering. The 16O16O formation in b is due to the oxygen deposition on the Pt surface from collision-induced dissociation of H216O+ (see text). The appearance of 18O sputtering peak ∼25 eV in d confirms that the surface is covered by 18O atoms, produced by in situ18O2 exposure.
Figure 3
Figure 3. Direct formation of O2 in collisions of normal water ions with an oxidized Pt surface.
Energy distributions of O2 ions produced from (a) H2O+/Pt(18O), (b) H2O+/Pt(16O) and (c) D2O+/Pt(16O). The Pt surface was exposed to 18O2 or 16O2 in situ at a background pressure of 5 × 10−8 torr. Results are shown for multiple incidence energies (E0) of the corresponding water ions, as indicated. Very weak signal from sputtered O2 appears as a second peak for E0>150 eV.
Figure 4
Figure 4. Proposed reaction mechanism and kinematics of direct O2 formation from water.
(a) Schematic depiction of the proposed Eley–Rideal reaction mechanism between energetic water ions and adsorbed O-atoms, producing highly excited oxywater (H2O–O* or D2O–O*), which undergoes delayed fragmentation to form HO2 (DO2) as the precursor for O2. (b) Ion exit energies of H2O+, O2 and H+ as a function of H2O+ incidence energy. The exit energy data of H2O–O* were estimated from the measured exit energies of O2 and H+ (see text). (c) Ion exit energies of D2O+, O2 and D+ as a function of D2O+ incidence energy. The exit energy data of D2O–O* were estimated (see text). All solid lines in b,c are linear fittings. The slopes for H2O+ and D2O+ are predicted from standard BCT. The slopes for H2OO* and D2OO* are calculated from a modified BCT model.

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